Filename: 215-update-min-consensus-ver.txt Title: Let the minimum consensus method change with time Author: Nick Mathewson Created: 15 Nov 2012 Status: Open 0. Overview This proposal suggests that we drop the requirement that authorities support the very old consensus method "1", and instead move to a wider window of recognized consensus methods as Tor evolves. 1. Background and Motivation When we designed the directory voting system, we added the notion of "consensus method" so that we could smoothly upgrade the voting process over time. We also said that all authorities must support the consensus method '1', and must fall back to it if they don't support the method that the supermajority of authorities will choose. Consensus method 1 is no longer viable for the Tor network. It doesn't result in a microdescriptor consensus, and omits other fields that clients need in order to work well. Consensus methods under 12 have security issues, since they let a single authority set a consensus parameter. In the future, new consensus methods will be needed so that microdescriptor-using clients can use IPv6 exits and ECC onion-keys. Rolling back from those would degrade functionality. We need a way to change the minimum consensus method over time. 2. Design I propose that we change the minimum consensus method about once per release cycle, or once per ever other release cycle. As a rule of thumb, let the minimum consensus method in Tor series X be the highest method supported by the oldest version that "anybody reasonable" would use for running an authority. Typically, that's the stable version of the previous release series. For flexibility, it might make sense to choose a slightly older method, if falling back to that method wouldn't cause security problems. For example, while Tor 0.2.4.x is under development, authorities should really not be running anything before Tor 0.2.3.x. Tor 0.2.3.x has supported consensus method 13 since 0.2.3.21-rc, so it's okay for 0.2.4.x to require 13 as the minimum method. We even might go back to method 12, since the worst outcome of not using 13 would be some warnings in client logs. Consensus method 12 was a security improvement, so we don't want to roll back before that. 2.1. Behavior when the method used is one we don't know The spec currently says that if an authority sees that a method will be used that it doesn't support, it should act as if the consensus method will be "1". This attempt will be doomed, since the other authorities will be computing the consensus with a more recent method, and any attempt to use method "1" won't get enough signatures. Instead, let's say that authorities fall back to the most recent method that they *do* support. This isn't any likelier to reach consensus, but it is less likely to result in anybody signing something they don't like. 3. Likely outcomes If a bunch of authorities were to downgrade to a much older version, all at once, then newer authorities would not be able to sign the consensus they made. That's probably for the best: if a bunch of authorities were to suddenly start running 0.2.0.x, consensing along with them would be a poor idea. 4. Alternatives We might choose a less narrow window of allowable method, when we can do so securely. Maybe two release series, rather than one, would be a good interval to do when the consensus format isn't changing rapidly. We might want to have the behavior when we see that everybody else will be using a method we don't support be "Don't make a consensus at all." That's harder to program, though.